Connections utilizing transmission lines formed by light guides are finding increasingly wide-spread use, particularly in the field of data-transmission. Besides remote communication by means of optical waveguides, such connections may also be used to provide interconnection between points which are normally connected by electrical connections.
An optical signal transmission line of the type in question generally consists of at least one optical waveguide with conversion means at either end. At one end of the optical waveguide, conversion means convert a signal to be transmitted, which is generally an electrical signal, into light energy and constitute emitting means. At the other end of the optical waveguide, conversion means convert the transmitted light energy back into a suitable signal, generally likewise in electric form, and constitute reception means. The optical waveguide is formed by at least one optical fiber and usually by a plurality of fibers which are combined into a fiber bundle.
An optical fiber is formed normally by a coaxial structure with a transparent core having a refractive index N1 surrounded by a layer of transparent cladding material having a refractive index N2 which is lower than N1. The refractive index values for glasses may vary between 1.5 and 1.485 for example. The diameter of the core may vary between a size close to the wavelength of the light when the lightguide to be produced is a monomode guide and approximately 80 .mu.m or more in the case of multimode guides. In preferred embodiments the diameter selected is in the range 25 to 45 .mu.m so as to reconcile transmitted power and pass-band characteristics while propogating only a fairly limited number of modes.
The transmission and reception means may be made small in size by using semiconductor components, such as gallium arsenide diodes for transmission and silicon diodes for reception. These semiconductor materials have normally considerable higher refractive indices than the optical waveguide.
If the refractive index of air is taken as 1, the value for the refractive index of silicon is on the order of 4, of gallium arsenide 3.6, and of the glass of the optical fiber 1.5.
The optical signal transmission line is thus formed with materials having different refractive indices along its optical path, as a result of which a part of the light energy transmitted is reflected and fails to reach the receiver.
The Fresnel formulae relating to normal reflection by transparent isotropic bodies give a value of [(n - 1)/(n + 1)].sup.2 for the reflection factor R of the boundary surface between two media, n being the relative index of one of the media with respect to the other, the transmittance being T = 4n/(n + 1).sup.2.
The reflection factor R becomes smaller the closer n approaches to 1, that is to say, the smaller the disparity between the indices. By way of example, the reflection factor R has the following values with the index values mentioned above and in the cases of the following interfaces:
glass/air interface, R.sub.G-A = 0.04 PA1 air/silicon interface, R.sub.A-Si = 0.36 PA1 air/gallium arsenide interface, R.sub.A-GaAs = 0.30 PA1 glass/silicon interface, R.sub.G-Si = 0.20 PA1 glass/gallium arsenide interface, R.sub.G-GaAs = 0.17
These last two values apply to embodiments where the glass of the fiber is in bonded contact with the semiconductor with no air interface.
To increase the performance of such optical communication systems, it has proved to be necessary to reduce propagation losses and in particular those due to reflections. Propagation losses in the fiber itself are negligible in the case of short-range connections, those of a few tens to a few hundreds of meters for example, and there are at the moment fibers in which attenuation is less than 2 dB per km and which in commercially available form provide an attenuation of 20 dB/km.
Taking the case of a light energy Eo, which is transmitted without appreciable losses along an optical waveguide and arrives at the exit interface, the major proportion of the energy is transmitted to the receiver and the remaining part E1 is reflected. This part E1 is propagated in the reverse direction to the entry interface where a further fraction E2 is reflected and travels back to the receiver, and so on. The amount of energy E2 which arrives at the receiver for the second time may be quite high. In the case of a short-range link having a gallium arsenide transmitter and a silicon receiver it may be 0.034 Eo, i.e. -32 dB, where the ends of the guide are bonded, and 0.13 Eo, i.e. -17 dB, with air interfaces. Under these conditions it is possible that the receiver will pick up this secondary signal, which represents a strong spurious signal. The result is that large disparities between the refractive indices at the ends are a serious restriction on the technical potentialities of such optical communication systems.